The present invention relates to a dimensionally stable paper and paper products, a production method for such paper, and uses of substances for conveying, modifying or improving dimensional stability of paper.
When cellulosic paper gets into contact with water, e.g. with water-based glues, wallpaper glue and poster glues, the cellulose fibers swell. This leads to a change in size of the paper by expansion into the wet state and shrinking into the dry state. For paper manufacturing on a Fourdrinier machine cellulose fibers are mainly aligned in the direction of production, i.e. in longitudinal direction. This alignment is favored by the pulling force exerted by the paper machine on the paper web. Swelling of cellulose fibers thus leads to dimensional changes, also called wet expansion, mainly in transverse direction of the paper web. Such expansion is generally not desirable; instead, dimensional stability in wet and air-dry conditions is considered favored.
Such dimensional changes are objectionable particularly for such applications where dimensional constancy in air-dry and wet conditions is mandatory. Particularly for wallpapers only a minimal change in dimensions is allowable. Wallpapers are normally adhered to a solid carrier in a wet state, either by applying a water-based glue to the wallpaper before pasting the then wet wallpaper to the solid carrier (“wet paste technique” or “Naβklebetechnik”), or by applying a water-based glue directly to the solid carrier and pasting the still dry wallpaper thereon (“Wandklebetechnik” or “paste-the-wall-technique”).
In either case the wallpaper will absorb water such that cellulose fibers of the paper will expand. After pasting of the wallpaper onto the solid carrier normally a further wallpaper is pasted adjacent to the first wallpaper. In such configuration, shrinkage of wallpaper webs will result in gap formation between formerly adjacent paper webs, which is highly undesirable. And if a wallpaper web is pasted on a solid carrier before the paper web is fully expanded, then the paper web will continue to expand on the solid carrier, leading to the undesirable development of folds, bubbles and raised edges.
Similar problems occur when using water-based glues for application of posters and when printing on paper using Coldset inks. However, also for other applications dimensional expansion is undesirable, e.g. for art paper, intaglio printing paper and paper for maps and cards.
To achieve dimensional stability it has been suggested to include synthetic fibers to produce a waterleaf paper (“Vliespapier”). As synthetic fibers have a low wet expansion the paper web is internally stabilized against significant wet expansion. However, a disadvantage of waterleaf paper production is the high production costs. To prevent synthetic fibers from spinning (“verspinnen”) the fibers have to be highly diluted in water. The headbox fiber suspension concentration generally is not more than 0.2 wt.-%. This results in a high energy demand for water circulation. Also, synthetic fibers are prone to dust generation and are generally not biologically degradable.
Also for papers other than waterleaf papers it has been tried to achieve improved dimensional stability by inclusion of synthetic fibers. For example WO 2008040635 A1 describes a method for production of a wallpaper substrate that can be peeled in a dry manner and having minimal wet expansion. In this method a multilayer substrate is produced having a lower layer for direction to a wall and an upper layer for direction to the room. A fibrous lower layer of a mixture of cellulosic and synthetic fibers is felted with a fibrous upper layer free of synthetic fibers. Again, production costs are disadvantageous, the method is considered cumbersome and the wallpaper substrate is prone to curling due to the differences in expansion capability of the layers.
Another wallpaper substrate is described in EP 1914087 A1; this wallpaper also has a nonwoven substrate comprising up to 50 wt.-% in synthetic fibers. Again, use of synthetic fibers is disadvantageous as described above.
It has been suggested in JP 2009096108 A to produce a wallpaper substrate comprising a layer of an expanded resin. A disadvantage of an expanded resin surface layer is that such papers can only be used in limited applications, and particularly cannot be generally used for wallpapers.
In a similar manner EP 98971 A2 discloses a structural wallpaper with a visible surface which is relief-like and may be painted over. The wallpaper consists of an embossed wallpaper substrate provided with an embossing on both faces, wherein to the reverse face of the embossed wallpaper a smooth paper, for example, waste paper, is glued. By using a water insoluble and water impenetrable glue areas are formed which do not absorb water and hence do not swell and expand. A disadvantage of such wallpaper is that the wallpaper substrate necessarily is embossed, limiting the usefulness of the wallpaper only to wallpaper applications and further limiting the possibilities of additional modifications of the wallpaper. Also the manufacturing method is rather laborious.
In WO 2005095712 A1 a wallpaper is described which is preferably free of synthetic fibers. The wallpaper consists of a number of layers, wherein a hydrophobic barrier layer is intended to separate the remaining wallpaper from a hydrophilic layer for gluing of the wallpaper to a wall. This was water absorption and wet expansion should be limited to the very layer used for gluing of the wallpaper to the wall. However, a disadvantage of this wallpaper is that the corresponding manufacturing method is cumbersome and the resulting wallpaper is inherently prone to curling.
It was thus an object of the present invention to provide a paper and paper product having low wet expansion in the wet state. The disadvantages described above should be avoided or reduced wherever possible. The paper should be useful in various fields of application, particularly for the production of wallpapers, but also for art papers, poster papers, intaglio papers and map and card papers. The paper should adhere to a wall when applied using wet paste technique or paste-the-wall-technique. In addition, the freshly applied paper should remain slidable for some time and have sufficient wet strength and dry strippability. The paper should have a good printability and preferably a low content of synthetic fibers. An object of the invention also was to provide manufacturing methods for producing such papers and paper products, and to provide uses of substances and mixtures for conveying, modifying or improving dimensional stability of paper and paper products.
According to the invention there is thus provided a dimensionally stable paper. The paper according to the invention comprises a raw paper impregnated at least on one face with an impregnation mixture comprising a sugar alcohol, urea and at least one substance selected from a polyalkylene glycol and glycerin. The paper according to the invention solves the above object of the invention and reduces or avoids the drawbacks of the prior art, particularly by exhibiting the features of preferred embodiments of the invention.